Bone is the most common site for prostate cancer metastasis; ~65-75% of patients with metastatic prostate cancer will develop bone metastases, with a median overall survival of 2-3 years following diagnosis. Metastases disrupt normal homeostasis between osteoclasts (bone resorption) and osteoblasts (bone formation), weakening the skeleton, causing an increased risk of bone fractures and severe pain. The tumor interacts with bone in a malignant crosstalk which acts to enhance tumor growth and worsen bone pathology. Our long-term objective is to develop a successful intervention for treating both the prostate tumor and the affected one tissue, eliminating the tumor metastases while restoring the bone to a normal functional state. The pathways promoting the malignant crosstalk between prostate tumors and bone involve signaling by cytokines interleukin (IL)-6 and IL-11 and are associated with more advanced disease and a poor prognosis. Our laboratory has developed strategies for disrupting these malignant cytokine signals by using inhibitory peptides targeting the IL-6 and IL-11 receptors. We propose a novel strategy to express these peptides in vivo following gene delivery as targeted and multifunctional 'propeptides'. These propeptides will target bone metastases and also contain dual therapeutic domains composed of IL6R and IL11R antagonist peptides and an osteogenesis-promoting peptide, osteostatin. The propeptides will be processed by gelatinases (MMP2/9) present at high levels in prostate tumor bone metastases, specifically releasing therapeutic peptides at the site of metastasis, where they will antagonize IL-6 and IL-11 signaling and promote bone restoration. The method of propeptide delivery will use ultrasound-enhanced or sonoporation gene delivery or 'sonodelivery'. We will test the hypothesis that optimized secretion and targeting of therapeutic propeptides will eliminate prostate tumor metastases and restore normal bone. The main goal of this proposal is to optimize expression and secretion of these targeted therapeutic propeptides following muscle sonodelivery, and enhance accumulation and specific peptide release at bone metastases to eliminate tumor metastases and restore bone. We propose the following specific aims to accomplish this goal: Aim 1. Optimize delivery of therapeutic propeptides to prostate tumor bone metastases, and Aim 2. Determine mechanisms of therapeutic efficacy of propeptides in vivo. For both Aims, detection of propeptide tumor delivery and efficacy of therapy will use microCT and optical imaging. We anticipate completion of this project will provide the foundations for a multifunctional, targeted, propeptide-based therapeutic for treating prostate cancer bone metastases. This project is innovative in that it will establish a novel therapeutic propeptide agent with a simple sonodelivery system that will target and treat both the tumor metastases and the bone. Of clinical significance, it is expected that this approach will enable improvement of bone-metastatic prostate cancer patient morbidity and quality of life and could be applicable to other diseases characterized by bone/tumor pathology.